The conventional image pickup apparatus such as digital cameras and cameras on mobile terminals is disposed with a continuous auto focusing function automatically adjusting a focus based on movement or changes of an object. The continuous auto focusing function may be divided into two mode, i.e., a monitoring mode monitoring image changes, and a focus determining mode seeking for an optimal focus of an image.
FIG. 1 illustrates images in the focus determining mode, and images through an edge filter. The monitoring mode anticipates image changes by using a time difference in the image changes or changes of edge values of images to implement the continuous auto focusing adjustment. That is, the monitoring mode discriminates that an image to be focused has been determined to perform the auto focusing adjustment, if an image change is equal to or greater than threshold value, and this condition is continuously maintained after lapse of a predetermined frame.
The focus determining mode uses an edge filter to extract an edge strength to horizontal and vertical directions, i.e., a gradient value of the edge value, and decides a point having the highest gradient value as a focus object position. That is, if the auto focus adjustment performance by the monitoring mode is determined in the focus determining mode, the edge filter is applied as shown in FIG. 1, to extract a gradient value (or FV: Focus Value) of the edge value to the horizontal and vertical directions and to determine a point having the highest value as a focus object position.
At this time, in a case a horizontal edge value is defined as HEV, a vertical edge value is defined as VEV, and a focus value is defined as FV, the FV can be obtained by an equation ofFV=√{square root over (HEV2+VEV2)}.
However, the conventional auto focusing method thus described has a drawback in that a time delay is generated until an object image to be focused is determined, because the focus determining mode is performed by discriminating that an image to be focused has been determined only if an edge strength of a relevant frame maintains a predetermined value or more for a predetermined period of time while an edge change of a video image is maintained at a predetermined threshold value or more in the monitoring mode.
Another drawback in the conventional auto focusing method is that the focus determining mode must be performed again if the video image is changed in the time delay generated by determination of object image to be focused, whereby unnecessary focus determining mode has to be repeatedly performed. Still another drawback in the conventional auto focusing adjustment method is that detection of image changes cannot be accurately performed based on image properties, whereby lots of time are consumed before the determination of the focus, if the image changes are measured by edge changes and stabilization time in the image monitoring mode for performing the focus determining mode.